In recent years, new energy-production techniques have attracted attention from the standpoint of environmental issues, and among these techniques a fuel cell has attracted particular interest. The fuel cell converts chemical energy to electric energy through electrochemical reaction of hydrogen and oxygen, attaining high energy utilization efficiency. Therefore, extensive studies have been carried out on realization of fuel cells for public use, industrial use, automobile use, etc.
Fuel cells are categorized in accordance with the type of employed electrolyte, and, among others, a phosphate type, a fused carbonate salt type, a solid oxide type, and a solid polymer type have been known. With regard to hydrogen sources, studies have been conducted on methanol; liquefied natural gas predominantly containing methane; city gas predominantly containing the natural gas; a synthetic liquid fuel produced from natural gas serving as a raw material; and petroleum-derived hydrocarbons such as LPG, naphtha, and kerosene.
Among the aforementioned petroleum-derived hydrocarbons, kerosene is easy to store and handle, and infrastructure of a supply system for kerosene is well developed (e.g., gas stations and shops). Thus, kerosene would advantageously serve as a hydrogen source for use in fuel cells for public use, automobile use, etc.
However, as compared with methanol and natural gas, petroleum-derived hydrocarbons problematically have a high sulfur content. In order to produce hydrogen from petroleum-derived hydrocarbons, the hydrocarbons are generally subjected to steam reforming or partial oxidation reforming in the presence of a reforming catalyst. In the course of such reformation, the reforming catalyst is poisoned by sulfur contained in the hydrocarbons. Therefore, from the viewpoint of catalyst life, the hydrocarbons are essentially subjected to desulfurization so as to control the sulfur content to generally 0.2 ppm by weight or lower.
Conventionally, a variety of studies have been carried out on methods for desulfurizing petroleum-derived hydrocarbons. Among them, one known method is hydro-desulfurization by use of a hydro-desulfurization catalyst such as Co-Mo/alumina or Ni-Mo/alumina and a hydrogen sulfide adsorbent such as ZnO, the hydro-desulfurization being effected under normal pressure to 5 MPa and at 200-400° C. However, the above method is directed to removal of sulfur by transformation into hydrogen sulfide through hydro-desulfurization performed under rigorous conditions, and controlling the sulfur content to 0.2 ppm by weight or lower is difficult. Thus, application of the method to desulfurizing petroleum-derived hydrocarbons for use in fuel cells is not encouraged.
From another viewpoint, there have been known nickel-based adsorbents and nickel-copper-based adsorbents serving as desulfurizing agents which can remove sulfur contained in petroleum-derived hydrocarbons through adsorption under mild conditions, without performing hydrorefining, thereby controlling the sulfur content to 0.2 ppm by weight or lower [Japanese Patent Publication (kokoku) Nos. 6-65602, 7-115842, and 7-115843 and Japanese Patent Application Laid-Open (kokai) Nos. 1-188405, 2-275701, 2-204301, 5-70780, 6-80972, 6-91173, and 6-228570 (nickel adsorbents), and Japanese Patent Application Laid-Open (kokai) No. 6-315628 (nickel-copper adsorbent)].
These nickel-based adsorbents and nickel-copper-based adsorbents are advantageously applied, as desulfurizing agents, to petroleum-derived hydrocarbons for use in fuel cells. However, the service life of these adsorbents is still unsatisfactory for use in practice, and conditions for designing adsorbents suitable for desulfurizing petroleum-derived hydrocarbon have not yet been elucidated. Particularly, the above nickel-copper-based adsorbents exhibit unsatisfactory performance for effective desulfurization.